FIG. 1 exemplarily shows a wireless communication system. Referring to FIG. 1, a wireless communications system 100 includes of a plurality of Base Stations (BSs) 110 and a plurality of Mobile Stations (MSs) 120. The wireless communications system 100 may include a homogeneous network or a heterogeneous network. The heterogeneous network refers to a network in which different network entities including an Institute of Electrical and Electronics Engineers (IEEE) 802.16e system, an IEEE 802.16m system, a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system, and a WiFi system coexist. The homogeneous network may be divided into a macro cell, a femto cell, a pico cell, a relay, and the like. Generally, the BS refers to a fixed station that communicates with MSs. The BSs 110a, 110b, and 110c provide services to specific geographical areas 102a, 102b and 102c, respectively. To enhance system performance, each of the specific geographical areas may be further divided into a plurality of subareas 104a, 104b, and 104c. Each of the subareas may be referred to as a cell, a sector, or a segment. In the IEEE 802.16e system, a cell identity is allocated based on the entire system. In contrast, a sector or segment identity is allocated based on a specific area in which each BS provides a service and has a value of 0 to 2. The MSs 120 are generally distributed in a wireless communication system and may be fixed or mobile. Each MS may communicate with one or more BSs at a certain time in uplink and downlink. The BS and the MS may perform communication using Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Single Carrier-FDMA (SC-FDMA), Multi Carrier-FDMA (MC-FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), or a combination thereof. In the present specification, uplink refers to a communication link from an MS to a BS and downlink refers to a communication link from a BS to an MS.